Papers used in a variety of applications, e.g. toweling and bags, require a high level of wet strength in order to maintain structural integrity under water-wet conditions. The use of resins, generally cationic resins, to improve the wet strength of paper is thoroughly discussed by H. H. Espy in "Pulp and Paper Manufacture, Third Edition", Volume 6, TAPPI, 1992, pages 65-83. This publication is incorporated herein in its entirety by reference.
Improving the wet strength of paper by use of cationic, water-soluble resins derived from reaction of epichlorohydrin with poly(diallylamine) is disclosed by Keim in U.S. Pat. Nos. 3,700,623 and 3,772,076, and by Van Eenam in U.S. Pat. Nos. 4,298,639 and 4,298,715, which four patents are incorporated herein by reference in their entirety.
The existence of azetidinium ions in wet strength resins prepared by reaction of epichlorohydrin with polyaminoamides, with polyalkyleneamines and with poly(diallylamine) is discussed by Espy in "Pulp and Paper Manufacture, Third Edition", Volume 6, TAPPI, 1992, pages 65-83, which is incorporated in its entirety by reference.
The epichlorohydrin-based wet strength resins referred to above are prepared by reaction of epichlorohydrin in aqueous solution with polymers containing secondary amino groups. Not all of the epichlorohydrin in the aqueous reaction mixture reacts with the amine groups to functionalize the polymer. Some of the epichlorohydrin remains unreacted, some reacts with water to form 3-chloropropane-1,2-diol, and some reacts with chloride ion to form dichloro-2-propanol, normally a mixture of 1,3-dichloro-2-propanol and 2,3-dichloro-1-propanol. The 2,3-dichloro-1-propanol is generally formed in very small amounts when compared to 1,3-dichloro-2-propanol. These organic chloride by-products are generally considered to be environmental pollutants, and increasing environmental concerns have created an interest in wet strength resins that have reduced levels of such by-products.
A drawback in the preparation of wet strength resins by reaction of epichlorohydrin with poly(diallylamine) is the concomitant formation of the epichlorohydrin by-products as indicated above. For example, the level of organochloride residues found in resins produced by the methods of U.S. Pat. Nos. 3,700,623 and 3,772,076 has been found to be about 8,000 ppm of unreacted epichlorohydrin, about 300,000 ppm of 1,3-dichloropropanol, about 99,000 ppm of 3-chloro-1,2-propanediol and about 160 ppm of 2,3-dichloro-1-propanol, all based on the dry weight of the resin.
Copending U.S. patent application Ser. No. 096,388, filed Jul. 26, 1993 discloses a process for reducing the levels of 1,3-dichloro-2-propanol and 2,3-chloro-1-propanol in the manufacture of polyaminoamide-epichlorohydrin resins comprising, conducting epichlorohydrin-polyaminoamide reaction in a mixture of water and an immiscible solvent for the epichlorohydrin.
Bull et al., in European Patent Application Publication No. 0 510 987A, disclose a process for dehalogenating the nitrogen-free, non-polymeric by-products of epihalohydrin reactions using enzymes.
Ehrhardt et al., in European Patent Application Publication No. 0 508 203A, disclose a process for decreasing epihalohydrin monomeric by-products in reaction of epihalohydrins with polyalkylene polyamines comprising addition of the amine to the epihalohydrin at low temperature.
Bower, in European Patent Application Publication No. 0 488 767A, discloses a two-step process for reducing the level of undesirable epichlorohydrin by-products in the reaction of epichlorohydrin with polyamines comprising reacting the epichlorohydrin with a polyamine first at a temperature of 25.degree.-40.degree. C. and then heating the mixture at 25.degree.-85.degree. C.
Miller et al., in U.S. Pat. No. 5,171,795, disclose a process for carrying out the reaction of epichlorohydrin with polyaminoamides that reduces the level of epichlorohydrin by-products, comprising reacting the polyaminoamide with epichlorohydrin at a temperature of about 20.degree.-60.degree. C. until at least 70% of the total charge of epichlorohydrin has reacted. The polymer solution is then treated with a mineral acid at 20.degree.-100.degree. C. followed by continuation of the reaction at 20.degree.-100.degree. C. to isomerize N-chlorohydrin groups to 3-hydroxyazetidinium chloride groups.
Gorzynski, in PCT WO 92/22601, discloses a process for preparation of epihalohydrin-based resins having reduced halogen content using a treatment with a basic ion-exchanger.
Amey, in PCT WO 93/21384, discloses preparation of polyamide-epichlorohydrin resins having low levels of free epichlorohydrin and related hydrolysis products using an adsorbent selected from ion-exchange resins, non-ionic polymeric resins, synthetic carbonaceous adsorbents, activated carbon, zeolites, silica, clays and alumina.
Devore et al., in U.S. Pat. Nos. 5,189,142 and 5,239,047, disclose methods for preparing aminopolyamide-epichlorohydrin resins containing low levels of total organic chlorine by varying process conditions.